Wednesday, March 14, 2012

Technical Corner - Keeping It Wheel

Wheels. Whats all the fuss about? Like many things in life wheels are one of those things that people obsess over. Wheels are an intense source of debate and a place where form and function meet and have bloody duels often resulting in shenanigans. They do of course have an important job to do, something to do with rolling. Click past the break for a brief history of the wheel you know and love today.

Rotaform NUE

Let me take you on a brief history tour before we settle on car wheels. Let's skip over the invention of the wheel by the Sumerians and ignor the Egyptians for coming up with spokes. I want you to meet John Boyd Dunlop.

John Boyd Dunlop.

John Boyd Dunlop was a Scotsman who trained as a veterinary surgeon and moved to Northern Ireland. By all accounts John Dunlop was quite a badass and liked to tinker in his garage when he wasnt saving animals. He invented the pneumatic tire (or tyre as he probably called it) in 1888 and this turned out to be rather a brilliant thing. At the same time Dunlop was fooling around with history, bicycles looked like this:

A Hipster's wet dream

Bicycles and horse drawn carts all had large wheels. They all had solid wheels and roads were not very good so to help them roll more smoothly they had to make the wheels really large. This huge front wheel made bicycles a tad dangerous but Dunlop solved this problem. With pneumatic tires to make the ride comfortable, wheels could be made in smaller diameters. The "safety bicycle" was born and its design is barely changed to this day. Dunlops patent was licensed in return for 1500 shares in the company that was formed to produce the new tires. Dunlop never made a fortune because his patent was later denied, another Scotsman Robert William Thompson had patented a similar design years earlier and had simply failed to find a commercial way to produce them. Regardless of who gets the credit, the modern wheel was born paving the way for cars and eventually hoonage and drifting.

Cheese also comes in the form of wheels. I really like cheese.

One way to make a wheel perform better is to make it lighter. As it turns out the weight of a wheel and tire influences the performance of a vehicle more than you might think. Wheels make up both rotating mass and unsprung mass and this makes the weight of wheels - per pound - very important to a cars performance. Its also worth pointing out that in most cases the wheel also has to be a heat sink to absorb the heat generated by braking. You can see why designers are willing to go a long way to reduce the mass of the wheel using exotic materials and manufacturing processes and gives rise to the other reason wheels are awesome:

They look important.

BBS Mesh

As materials and manufacturing technology has changed so have the wheels. Over the years spokes gave way to steel wheels and those have been replaced with aluminum alloy wheels. The jump to aluminum alloy is so significant that cars made with the cheaper steels wheels use plastic hub caps designed to make them look like the more expensive aluminum wheels. Its so ingrained that if we take the plastic hub caps off a car with steel wheels the car almost appears to be damaged or broken in some way. There is no denying it, wheels make a dramatic difference to the way a car looks.

Rotaform RWB

In the 1960s the need to provide space for larger braking systems created a demand for larger wheels. 13 and 14 inch wheels gave way to 15 inch wheels. To maintain adequate tire clearance and the correct outer tire diameter the concept of plus sizing took hold. The performance trend abated in the 1970s and 1980s as buyers focused on economy but when the 1990s came around with cheap gasoline the performance oriented buyers opened their wallets and the plus sizing trend took off again.

Work Equips

Sidewall profiles shrank and gave drivers more control and response. 16 inch wheels became the standard high performance wheel, then 17s and 18s and now some OEM wheels are 19 or even 20 inches in diameter to accommodate huge brake systems. The plus sizing trend lead to various custom car subcultures the most obvious today would be the "Donk" look. Other size and fitment trends have evolved some regional, some global with some favoring form over function with extreme results.

wtf?

If you're looking for some new shoes for your car, how do you know what a good wheel is? How they are made defines the properties of the wheel and requires us to take a brief look at aluminum processing.

There's a few different ways of getting aluminum into the shape of a wheel. Lets take a look at the common methods.

(CNC HRE)

First of all its important to recognize that wheels are not made of pure aluminum or aluminium (if you live outside the United states). Pure aluminum isnt very strong and corrodes easily. To make it stronger it is alloyed with other ingredients. Typical alloys contain zinc, copper, magnesium, manganese and silicon.

Casting:
Casting is a very cost effective way of making wheels in large numbers. There's very little material waste because you fill the mold and machine very little material off. There's also less work to do because the wheel takes the shape of the mold itself. Simply melt the alloy, pour, cool, finish machine a few spots and done. Sound great, but there are problems. To cast with aluminum we need to add a lot of silicon to the alloy to reduce shrinkage while the liquid alloy cools. Without this the casting would be full of internal cracks as some parts cool rapidly (touching the surface of the cooler mold). The addition of silicon weakens and softens the resulting alloy.

(Sand Casting, in China)

To mitigate these effects some clever folks have evolved some pressure casting methods that force the alloy into the mold much faster to avoid cracking, meaning less silicon can be used, making a stronger alloy and a lighter wheel. Using pressure to force the molten alloy in is better than using gravity, but it still requires a compromise to the material properties.

In high performance circles (see what I did there?) it is preferred to forge a wheel. Forging is basically a process of hammering the material into the right shape using large dies and rollers. Its usually finished off with some machining work as well. Forging means stronger alloys can be used and result in lighter wheels. The tooling for forging a wheel is expensive though so one way to create lots of different offsets with one set of tooling is to make 2 or 3 piece wheels that bolt or weld together with the required dimensions.

It is possible to create a one off wheel design without creating any special casting tooling or forging dies. Its possible to take a lump of aluminum alloy and machine a wheel using CNC machining techniques. This means wasting a lot of material (even though the chips are recycled) and means that one wheel can cost a lot of money. Also machining interrupts the grain flow of the material so typically these are not as strong and light as a forged wheel. CNC wheels are therefore mainly done for custom designs that will not be repeated and wouldn't be economical to produce a tool for.

With all of these designs its possible to trade off strength for lighter weight. Colin Chapman liked to minimize weight even if it meant failure, so long as it lasted long enough to finish the race. With this in mind we shouldn't assume that one type of wheel is stronger or weaker than any other, strength is up to the individual designer, the weight will be a result of the design and the material used.

2 comments:

Also to keep in mind, if you have too shallow a profile on your tires you deteriorate ride quality, which is bad in a daily driver. That and somehow the tire companies realized they can sell you less rubber for a higher price.